A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to provide a high throughput, i.e. pattern a large number of substrates per hour, lithographic apparatus normally employ powerful radiation sources. Therefore, although the elements of the projection system, which project the desired pattern onto the substrate, have a high transmissivity or reflectivity, the amount of radiation absorbed by them is not negligible and results in significant heating of the elements, even with a highly effective temperature control system for the projection system. Since this heating is non-uniform it may result in distortion of the shapes of the elements sufficient to cause significant distortion of the projected image. This is a particular problem of projection systems using refractive lenses and is often referred to generally as lens heating, though it also occurs with reflective and catadioptric systems. The term lens heating herein is used generally to refer to heating of one or more optical elements of a projection system, whether those one or more optical elements are transmissive, reflective, or both or whether those one or more optical elements are one or more lenses, one or more mirrors, etc.
Lens heating effects may be calculated in advance using a mathematical model that calculates, for a given pattern to be projected and a given illumination mode, where the projection beam will be localized in the projection system, the heating that will be caused and the resulting aberrations of the projected image. Adjustable optical elements provided in the projection system, for this and other corrections, may be used to introduce compensating aberrations so that the effect of lens heating is at least mitigated. Adjustable optical elements may allow for effective correction of aberrations describable by low order Zernike polynomials, up to Z16. An example of an adjustable optical element, intended for correction of magnification, astigmatism and field curvature, is disclosed in European patent application publication EP 0 660 169 A and comprises two cylindrical lenses with opposite power that can be relatively rotated.